MPC8347VRAGDB datasheet(90/99 Pages) FREESCALE

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MPC8347EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 12

Freescale Semiconductor

Thermal

Shin-Etsu MicroSi, Inc.

888-642-7674

10028 S. 51st St.

Phoenix, AZ 85044

Internet: www.microsi.com

The Bergquist Company

800-347-4572

18930 West 78th St.

Chanhassen, MN 55317

Internet: www.bergquistcompany.com

20.3

Heat Sink Attachment

When heat sinks are attached, an interface material is required, preferably thermal grease and a spring clip.

The spring clip should connect to the printed-circuit board, either to the board itself, to hooks soldered to

the board, or to a plastic stiffener. Avoid attachment forces that can lift the edge of the package or peel the

package from the board. Such peeling forces reduce the solder joint lifetime of the package. The

recommended maximum force on the top of the package is 10 lb force (4.5 kg force). Any adhesive

attachment should attach to painted or plastic surfaces, and its performance should be verified under the

application requirements.

20.3.1

Experimental Determination of the Junction Temperature with a

Heat Sink

When a heat sink is used, the junction temperature is determined from a thermocouple inserted at the

interface between the case of the package and the interface material. A clearance slot or hole is normally

required in the heat sink. Minimize the size of the clearance to minimize the change in thermal

performance caused by removing part of the thermal interface to the heat sink. Because of the experimental

difficulties with this technique, many engineers measure the heat sink temperature and then back calculate

the case temperature using a separate measurement of the thermal resistance of the interface. From this

case temperature, the junction temperature is determined from the junction-to-case thermal resistance.

TJ = TC + (RθJC × PD)

where:

TJ = junction temperature (°C)

TC = case temperature of the package (°C)

RθJC = junction-to-case thermal resistance (°C/W)

PD = power dissipation (W)

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